Electrical measuring apparatus for providing an output signal at an output branch characteristic of the relationship of impedances presented at first and second side branches at the frequency of energy applied at an input branch

ABSTRACT

A radio frequency signal source delivers a signal to an input branch through a harmonic filter to provide an output signal on an output branch indicated by indicating means representative of the difference in impedance between a standard impedance connected to one side branch and an unknown impedance connected to the other side branch. A pair of equal resistances connect the high terminal of the input branch to the respective high terminals of the respective side branches. A higher resistor intercouples the high terminals of the side branches so that when the side branches are terminated in their characteristic impedance, typically 50 ohms, the impedance presented at the input branch is substantially equal to the input branch characteristic impedance, typically 50 ohms. Means including isolating resistors and a bridge circuit couples the side branch high terminals to the output branch high terminal, all four branches having low, or grounded terminals, that are interconnected.

United States Patent [72] inventor Andrew Alford Winchester. Mass. 01890[21] App]. No. 756,734 [22] Filed Aug. 23, 1968 (45] Patented July 27,I971 Continuation-impart of application Ser. No. 391.337, Aug. 21. 1964,now abandoned.

[S4] ELECTRICAL MEASURING APPARATUS FOR PROVIDING AN OUTPUT SIGNAL AT ANOUTPUT BRANCH CHARACTERISTIC OF THE RELATIONSHIP OF IMPEDANCES PRESENTEDAT FIRST AND SECOND SIDE BRANCHES AT THE FREQUENCY OF ENERGY APPLIED ATAN INPUT BRANCH 4 Claims, 2 Drawing Figs.

[52] U.S. CI 324158 A, 324/57 [5!] Int.Cl. ..G0ln 27/04 [50] Field ofSearch 324/57 B, 57, S8 A, 58 B, 119

[56] References Cited UNITED STATES PATENTS 2,394,892 2/1946 Brown324/57 UX 2,521,522 9/1950 Keitley 324/1 [9 X 3,278,840 10/1966 Wilson324/57 OTHER REFERENCES Montgomery. Technique Of Microwave Measurement.in M.l T Radiation Lab Series Vol ll, (1947) pp. 8l 85 (Copy in group250 class 324 sub 104) Primary Examiner- Edward E. Kubasiewicz Atromey-Wolf, Greenfield and l-lieken ABSTRACT: A radio frequency signal sourcedelivers a signal to an input branch through a harmonic filter toprovide an output signal on an output branch indicated by indicatingmeans representative of the difference in impedance between a standardimpedance connected to one side branch and an unknown impedanceconnected to the other side branch. A pair of equal resistances connectthe high terminal of the input branch to the respective high terminalsof the respective side branches. A higher resistor intercouples the highterminals of the side branches so that when the side branches areterminated in their characteristic impedance, typically 50 ohms, theimpedance presented at the input branch is substantially equal to theinput branch characteristic impedance, typically 50 ohms. Meansincluding isolating resistors and a bridge circuit couples the sidebranch high terminals to the output branch high terminal, all fourbranches having low, or grounded terminals, that are interconnected.

TJN KNOWN IMPEDANCE lNDlCATlNG MEANS STANDARD IMPEDANCE PATENTEU M27197!3596,1 75

SIGNAL .1 SOURCE IO H A IC r FIG! UNKNOWN IMPEDANCE STANDARD IMPEDANCE aN 0| CATI NG MEANS FIGZ //V VE N TOR AN DREW ALFORD 9fogvi 'em ATTORNEYSELECTRICAL MEASURING APPARATUS FOR PROVIDING AN OUTPUT SIGNAL AT ANOUTPUT BRANCH CHARACTERIS'IIC OF THE RELATIONSHIP OF IMPEDANCESPRESENTED AT FIRST AND SECOND SIDE BRANCHES AT THE FREQUENCY OF ENERGYAPPLIED AT AN INPUT BRANCH This application is a continuation-in-part ofapplication Ser. No. 391,337 filed Aug. 21, 1964 now abandoned.

BACKGROUND OF THE INVENTION The present invention related in general toelectrical measuring and more particularly concerns novel techniques forderiving an output signal representative of the relationship between apair of impedances over a wide frequency range at VHF, HF, UHF ormicrowave frequencies. Measuring apparatus according to the invention isrelatively easy andinexpensive to fabricate, yet provides remarkableaccuracy over a wide frequency range.

It is an important object of this invention to compare a pair ofimpedances over a relatively wide frequency range.

It is another important object of the invention to achieve the precedingobject by providing a signal representative of the difference between apair of impedances over a wide frequency range.

It is still another object of the invention to achieve the precedingobjects with apparatus which is relatively inexpensive and easy tofabricate while carrying out measurements with a degree of accuracy.

It is still another object of the invention to achieve the operate byunskilled personnel.

SUMMARY OF THE INVENTION According to the invention an electricalcircuit provides an output signal at an output branch characteristic ofthe relationship of impedances presented at first and second sidebranches at the frequency of energy applied at an input branch. Eachbranch has a high terminal and a low terminal. Means, such as a shield,intercouple the low terminals to maintain them at substantially the samereference potential, typically ground potential. Means including a firstresistance intercouple the first side branch and the input branch highterminals. Means including a second resistance intercouple the secondside branch and the input branch high tenninals. Means including a.third resistance intercouple the first side branch and the second sidebranch high terminals. Detecting means intercouple the first and secondside branch high ter-- minals with the output branch high terminal forproviding a signal on the output branch high terminal representative ofthe difference of potential between the first side branch and the secondside branch high terminals. The input branch may be characterized by aninput branch characteristic impedance. Preferably, the first, second andthird resistances coact to present the input branch characteristicimpedance to the input branch when the side branches are each terminatedin the side branch characteristic impedances while then being arrangedto transfer equal amounts of energy from the input branch to each of theside branches to then produce substantially no difference in potentialbetween the first side branch and the second side branch high terminals.

In a specific form of invention, the detecting means comprises a bridgecircuit having four unilaterally conducting devices cascaded to definefirst, second, third and fourth consecutive junctions. Means maintainthe first junction substantially at reference or ground potential. Meansmaintain the output branch high terminal substantially at the potentialof the third junction. Means including a fourth resistance couple thefirst side branch high terminal to the second junction, and meansincluding a fifth resistance couple the second side branch high terminalto the fourth junction. The resistance of each of the fourth and fifthresistances is preferably high compared to that of the side branchcharacteristic impedance to prevent the bridge circuit from appreciablyaffecting the impedance presented to the side branches and the inputbranch.

Numerous other features, objects and advantages of the invention willbecome apparent from the following specification when read in connectionwith the accompanying drawing, in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a combined block-schematicrepresentation of the complete measuring system embodying the inventiveconcepts; and

FIG. 2 shows a schematic circuit diagram of a modified measuring bridge.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference now to FIG.1 of the drawing, there is shown a combined block-schematic diagramillustrating the arrangement of the complete measuring systemincorporating the principles of the invention. A signal source 10,typically a modulated or unmodulated radio frequency signal source,delivers a signal to the input branch 12 through harmonic filter 11 toprovide an output signal on output branch 14 indicated byindicating'means 16, which may be an ordinary meter, representative ofthe difference in impedance between standard impedance 18, connected toside branch II, and unknown impedance 20, connected to side branch I, atthe frequency then emitted by signal source 10.

Each of the four branches l2, 14, I and II include low terminals 30, 32,34 and 36, respectively. Typically these branches are coaxial as shownand may have characteristic impedances of the order of 50 ohms. The lowterminals are typically intercoupled by the conductive casing 38 whichmay typically be a conducting shield in which the components areenclosed, and the various branches are brought out through conventionalcoaxial terminal pairs.

A first resistance 40 intercouples high terminals 22 and 26 of inputbranch 12 and side branch I. A second resistance 42 intercouples highterminals 22 and 28 of input branch 12 and side branch II, respectively.A third resistance 44 intercouples high terminals 26 and 28 of sidebranches I and II. Resistances 40 and 42 have substantially the samevalues. Resistance 44 is such that when side branches I and II areterminated in their characteristic impedance, typically 50 ohms, theimpedance presented at the input branch 12 is substantially equal to theinput branch characteristic impedance, typically also 50 ohms.

Detecting means comprises a diode bridge 46 with four cascaded diodesintercoupling the four junctions 48, 50, 52 and 54. The first junction48 is connected to case 38. The third junction 52 is connected to highterminal 24 of output branch 14. The second junction 50 is coupled tohigh termmal 28 of side branch II by a resistor 56 and a capacitor 58.The fourth junction 54 is coupled to high terminal 26 of side branch Iby resistor 60 and capacitor 62. A bypass capacitor 64 is connectedbetween the fourth junction 52 and ground. The resistance of resistors56 and 60 are preferably larger than the characteristic impedances ofthe side branches so that the bridge circuit impedance which depends onthe impedances of the diodes has little effect on the impedancepresented by the network as seen looking into either of the two sidebranches.

Having described the arrangement of the system, its mode of operationwill be described. It is first convenient to assume that both sidebranches I and II are terminated in equal resistances corresponding tothe characteristic impedance of these side branches. Energy applied toinput branch 12 then divides equally between the two side branches l andII so that the signal amplitudes on high terminals 26 and 28 aresubstantially equal to produce substantially no difference in potentialbetween junctions 50 and 54 of bridge circuit 46. If the unknownimpedance 20 is higher than that of standard impedance 18, the signalamplitude on high terminal 26 is greater than that on high terminal 28.There will also be a corresponding difference in potential between thejunctions 50 and 54 of bridge circuit 46. and consequently acorresponding difference of low frequency potential between junction 52and the outer case 38. The difference of potential between junction 52and the case 38 will also exist when the unknown impedance 20 is smallerthan the standard impedance I8. The magnitude of this potential isrelated to the magnitude of the reflection coefficient of the unknownimpedance with respect to the standard impedance. The indicating means16 may be calibrated to indicate reflection coefficient magnitude orVSWR directly.

It can be shown that if the indicating device 16 is to indicate the VSWRindependently of the position of the standing wave along the line, it isnecessary that the impedances seen looking into the side branches besubstantially equal to the characteristic impedance of the transmissionline connected to, for example, branch 34, 26 along which the VSWR is tobe measured.

We now consider the impedance seen looking into port 26, 34, that is,the impedance seen looking back from the unknown into the bridge. Itwill be assumed that the bypass condensers 58, 62 and 64 aresufficiently large so that their impedances are small in comparison withthe resistances 60 and 56. It will also be assumed that the diodes inthe balanced rectifier bridge are small and that their impedances areessentially resistive. Let the resistance seen looking from capacitor 58into the diode bridge be R Let the resistance of resistor 56 be R theresistance of resistor 42 be R,, and the resistance of resistor 44 be R,Let it be assumed that the bridge is symmetrical with respect to theline drawn from 22 to 24 so that resistor 40 is equal to resistor 42,resistor 60 is equal to resistor 56. The resistance seen looking intothe diode bridge from junction 54 of value R, is the same as theresistance seen looking into the diode bridge from junction 50, and theresistance from each of side terminals 26 and 28 to junction 48 is R Thepurpose of the following calculations is to derive the resistance seenlooking into the side port, such as 26, 34 in FIG. 1.

To simplify the calculations, consider a circuit in which a generator ofvoltage E and source impedance Z terminates 24, 32, and respectiveimpedances of Z, terminate 28, 36 and 22, 30. Voltage V, on andconsequently current i through terminal 22 can be easily found becausethere is no current I through R, (because of the equality of thevoltages on the two sides of the circuit).

The resistance presented to the generator of voltage E is R"=%(R +R')where R'=Z,R,JZ,,+R the parallel combination of Z, and R By applying theprinciples of reciprocity we may interchange current i, with thegenerator with the result that V is the voltage at the junction ofresistors 40 and 42 and the current flowing from terminal 26 is of value1', with the equivalent resistance R connected from terminal 26 to thecase 38. V, is then the voltage at the junction of the resistors 40 and42 of value R,.

If we assign a current of value i, flowing through resistors 42 from thejunction of resistors 42 and 44 to the junction of resistors 40 and 42and a current of value 1', through resistor 44 from the junction ofresistors 42 and 44 to the junction of resistors 40 and 44, applyingKirchoffs law about the loop consisting of resistors 40, 42 and 44yields R.(2i.i.i-R.i.=0. (1) the voltage across resistor 40 being R M-iThe potential V, at the junction of resistors 40 and 44 is s= 2 l o rthe algebraic sum of the currents flowing from that junction into theparallel combination of resistances of value 2,, and R this parallelresistance being R.

Also,

where u =R'/Z, and q=Z,,/ R, (7) (7) When the side input I is matched to2,,

From (8) and (6) we can find u for a given value of q. For example, inthe specific embodiment of the invention with resistors 40 and 42 ofresistance R 50 ohms and resistor 44 of resistance R 60 ohms and Z 50ohms, q=l/ 1.2 or 0.835.

Therefore, R ==769 ohms.

In the specific example resistors 56 and 60 are 400 ohms, and each sidebranch is perfectly matched if the diode or bolometer bridge impedanceas seen from each side were 369 ohms. This is approximately what thediode bridge impedance is for low currents and small diodes, such as thetype lN23 or 1 N270 diodes.

The arrangement of the present invention includes decoupling resistorssuch as 60 and 56 which reduce the effect of the variable impedances ofthe diodes so that the side branch impedances remain substantiallyconstant even when the power of the RF signal from source 10 is varied.

The rectifying arrangement is deliberately made balanced so that itwould have equal effects on the RF impedances of the branches. This isdesirable even though the decoupling re sistors referred to above wouldreduce the unbalancing effects dissymetry, if any, to a lower value.

In an exemplary embodiment of the invention, resistors 40 and 42 are 50ohms, resistor 44 is 60 ohms, resistors 56 and 60 are 400 ohms,capacitor 58 and capacitor 62 are picofarads, capacitor 64 is 200picofarads for frequency range 100 mc.-l000 me. The diodes of bridgecircuit 46 are type lN23 or 1N270 diodes. The input, output and sidebranches comprise conventional Type N coaxial terminals.

In an alternate arrangement (shown in FIG. 2) two diodes in the diodediamond are replaced with two bolometers and the remaining two withresistors (See FIG 2). In this case the oscillator (source 10) must beamplitude modulated with a square wave or sine wave at say 1000 c.p.s.

There has been described novel techniques for accurately comparingimpedances over a wide frequency range with relatively simple apparatuseasily operated by unskilled personnel. It is apparent that thoseskilled in the art may now make numerous modifications of and departuresfrom the specific embodiment described herein without departing from theinventive concepts. Consequently, the invention is to be construed aslimited solely by the spirit and scope of the appended claims.

What I claim is:

1. In an electrical circuit, operative over a broad band of frequencies,for providing an output signal related to the difference in impedancepresented by an unknown impedance connected to a first side branch by atransmission line and a standard impedance connected to a second sidebranch, the

electrical circuit being of the type having an input branch foraccepting an AC input signal,

a first resistor connected between the input branch and the second sidebranch,

a second resistor connected between the input branch and the second sidebranch,

at third resistor connected between the side branches to' form a deltacircuit with the first and second resistors,

detecting means for detecting the difference in electrical potentialbetween the side branches, and means coupling the detecting means to theside branches, the improvement wherein the means coupling the detectingmeans to the side branches comprise resistors which provide appreciableimpedance at the higher frequencies in the broad band where theimpedance of the detecting means falls to a low value, and r the thirdresistor in the delta circuit differs from the value of the other tworesistors in the delta circuit, the value of the third resistors beingsuch as to cause the impedance seen looking into the first side branchto match, over a large portion of the broad band, the characteristicimpedance of the transmission line con necting the unknown impedance tothe first side branch. 2. The electrical circuit according to claim 1,wherein the improvement further is characterized in that the standardimpedance has a value matching the characteristic impedance of thetransmission line which is connected to the first side branch. 3. Theelectrical circuit according to claim 1, the improvement being furthercharacterized by a full wave rectifying bridge in the detecting means,the rectifying bridge being balanced with respect to the the sidebranches to cause the rectifying bridge to have an equal efiect upon theimpedance seen looking into each side branch, and an indicator connectedacross diagonally opposite junctions of the bridge to cause theindicator to respond to the rectified current flowing in the bridge. 4.The electrical circuit according to claim 1, the improvement beingfurther characterized by a bridge in the detecting means, the bridgebeing formed by four resistive arms, the resistive elements in two armsof the bridge being bolometers, and the bridge being balanced withrespect to the side branches to cause the bridge to have an equal effectupon the impedance seen looking into each side branch.

1. In an electrical circuit, operative over a broad band of frequencies, for providing an output signal related to the difference in impedance presented by an unknown impedance connected to a first side branch by a transmission line and a standard impedance connected to a second side branch, the electrical circuit being of the type having an input branch for accepting an AC input signal, a first resistor connected between the input branch and the second side branch, a second resistor connected between the input branch and the second side branch, a third resistor connected between the side branches to form a delta circuit with the first and second resistors, detecting means for detecting the difference in electrical potential between the side branches, and means coupling the detecting means to the side branches, the improvement wherein the means coupling the detecting means to the side branches comprise resistors which provide appreciable impedance at the higher frequencies in the broad band where the impedance of the detecting means falls to a low value, and the third resistor in the delta circuit differs from the value of the other two resistors in the delta circuit, the value of the third resistors being such as to cause the impedance seen looking into the first side branch to match, over a large portion of the broad band, the characteristic impedance of the transmission line connecting the unknown impedance to the first side branch.
 2. The electrical circuit according to claim 1, wherein the improvement further is characterized in that the standard impedance has a value matching the characteristic impedance of the transmission line which is connected to the first side branch.
 3. The electrical circuit according to claim 1, the improvement being further characterized by a full wave rectifying bridge in the detecting means, the rectifying bridge being balanced with respect to the the side branches to cause the rectifying bridge to have an equal effect upon the impedance seEn looking into each side branch, and an indicator connected across diagonally opposite junctions of the bridge to cause the indicator to respond to the rectified current flowing in the bridge.
 4. The electrical circuit according to claim 1, the improvement being further characterized by a bridge in the detecting means, the bridge being formed by four resistive arms, the resistive elements in two arms of the bridge being bolometers, and the bridge being balanced with respect to the side branches to cause the bridge to have an equal effect upon the impedance seen looking into each side branch. 